Pressure swing adsorption process and apparatus for purifying a hydrogen-containing gas stream

[0001] This application is a Continuation of copending International Application No. PCT/US2016/051572 filed Sep. 14, 2016 which application claims benefit of U.S. Provisional Application No. 62/219,416 filed Sep. 16, 2015, the contents of which cited applications are hereby incorporated by reference in their entirety.

[0002] The technical field generally relates to apparatuses and processes for purifying an impure gas stream predominantly containing hydrogen. More particularly, the technical field relates to pressure swing adsorption (PSA) apparatuses and processes for purifying impure gas streams predominantly containing hydrogen.

[0003] Pressure swing adsorption (PSA) processes provide an efficient and economical process for separating a multicomponent gas stream that contains at least two gases that have different adsorption characteristics. One of the gases may be preferentially adsorbed and can be an impurity that is separated from the other gas, which may be taken off as product. Alternatively, the gas that is preferentially adsorbed can be the desired product, which is separated from the other gas. For example, it may be desired to remove carbon dioxide, carbon monoxide, and light hydrocarbons from a hydrogen-containing feed stream to produce an impurity-depleted hydrogen (>99.9 mol % H2) stream for a hydrocracking or hydrotreating process where the impurities, especially carbon monoxide, could adversely affect the catalyst or the reaction.

[0004] In pressure swing adsorption processes, the multicomponent gas stream is typically fed to one or more adsorption beds at an elevated pressure to promote adsorption of at least one component, while at least one other component (for example, hydrogen) passes through the adsorption bed. At a defined time, feed to the adsorption bed is terminated and the adsorption bed is depressurized in one or more concurrent depressurization steps wherein pressure is reduced to a defined level that permits the separated, less-strongly adsorbed component or components to be withdrawn from the adsorption bed without significant desorption of the preferentially adsorbed components. Then, the adsorption bed is depressurized in a counter-current depressurization step (blowdown step) wherein the pressure in the adsorption bed is further reduced by withdrawing desorbed gas countercurrent to the direction of the multicomponent feed stream. Following the blowdown step, the bed is purged in a counter-current direction at a low pressure with a purified hydrogen stream to further desorb impurities, thereby creating a low-pressure tail gas stream. In multi-bed adsorption units, there are typically additional steps, and those noted above may be done in stages.

[0005] It is particularly desirable to minimize the amount of carbon dioxide and carbon monoxide in the impurity-depleted hydrogen streams. Conventionally, an activated carbon layer is used to remove carbon dioxide. However, still undesirable quantities of CO₂ migrate through the activated carbon layer. The migrated CO₂ interferes with the removal of other undesirable impurities such as CO and N₂ in the subsequent layers. Further, U.S. Pat. No. 4,775,396 teach using zinc or rare-earth exchanged faujasite molecular sieve for removing CO₂. However, this would entail significant cost.

[0006] Accordingly, it is desirable to provide methods and apparatuses for improving the removal of carbon dioxide in steam reforming based hydrogen PSA units and achieve net gain in performance. Further, it is desirable to provide a cost-effective method and apparatus that protects the sections subsequent to CO₂ removal section in a PSA unit from CO₂ migration and hence improve their efficiency in removing other impurities. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the subject matter.

[0007] Various embodiments contemplated herein relate to apparatuses and processes for purifying impure gas streams. The exemplary embodiments taught herein include apparatuses and processes for purifying impure gas streams predominantly containing hydrogen using pressure swing adsorption (PSA) apparatuses and processes.

[0008] In accordance with an exemplary embodiment, a multilayer adsorbent bed is provided for purifying a feed stream comprising CO₂, at least one of N₂, H₂O, CH₄ and CO, and more than 50 mole % hydrogen comprising a first adsorbent section, a second adsorbent section and a third adsorbent section. The first adsorbent section comprises an activated carbon layer and occupies a volume fraction of about 20 to about 50% of the multilayer adsorbent bed. The second adsorbent section comprises a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.5 to 8.0 and occupies a volume fraction of about 10 to about 25% of the multilayer adsorbent bed. The third adsorbent section comprises a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.0 to 1.5 and occupies a volume fraction of about 30 to about 60% of the multilayer adsorbent bed. Further, the second adsorbent section is present between the first adsorbent section and the third adsorbent section.

[0009] In accordance with another exemplary embodiment, a process is provided for purifying a feed stream comprising CO₂, at least one of N₂, H₂O, CH₄ and CO, and more than 50 mole % hydrogen, the process comprising passing the feed stream at an adsorption pressure of about 1500 kPa to about 3500 kPa, through a multilayer adsorbent bed comprising a first adsorbent section, a second adsorbent section downstream from the first adsorbent section and a third adsorbent section downstream from the second adsorbent section. The first adsorbent section comprises an activated carbon layer, the second adsorbent section comprises a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.5 to 8.0 and the third adsorbent section comprises a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.0 to 1.5. At least one of the N₂, CO₂, CH₄ and CO is adsorbed from the feed stream and a purified hydrogen product is recovered from the multilayer adsorbent bed.

[0010] In accordance with yet another exemplary embodiment, an apparatus is provided for purifying a feed stream comprising CO₂, at least one of N₂, H₂O, CH₄ and CO, and more than 50 mole % hydrogen. The apparatus comprises a feed end for the introduction of the feed stream thereto, a product end for the recovery of purified feed stream therefrom and a multilayer adsorbent bed comprising a first adsorbent section, a second adsorbent section downstream from the first adsorbent section and a third adsorbent section downstream from the second adsorbent section. The first adsorbent section comprises an activated carbon layer, the second adsorbent section comprises a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.5 to 8.0 and the third adsorbent section comprising a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.0 to 1.5.

[0015] As used herein, the term “stream” can include various hydrocarbon molecules and other substances.

[0016] As depicted, process flow lines in the FIGURES can be referred to interchangeably as, e.g., lines, pipes, feeds, gases, products, discharges, parts, portions, or streams.

[0017] The term “communication” means that material flow is operatively permitted between enumerated components.

[0018] The term “downstream communication” means that at least a portion of material flowing to the subject in downstream communication may operatively flow from the object with which it communicates.

[0019] The term “upstream communication” means that at least a portion of the material flowing from the subject in upstream communication may operatively flow to the object with which it communicates.

[0020] The term “predominantly” means a majority, suitably at least 50 mol % and preferably at least 60 mol %.

[0021] The term “feed stream” as used herein includes the “hydrocarbon stream”, as well as it may include a carrier fluid stream such as hydrogen, carbon dioxide, carbon monoxide, nitrogen, or any streams used in the systems and the processes described herein.

[0022] The term “adsorption” as used herein encompasses the use of a solid support to remove atoms, ions or molecules from a gas or liquid.

[0023] The term “Pressure swing adsorption (PSA)” refers to a process where a contaminant is adsorbed from a gas when the process is under a relatively higher pressure and then the contaminant is removed or desorbed thus regenerating the adsorbent at a lower pressure.

[0024] The term “atomic ratio” may be used interchangeably with “mole ratio”.

[0025] The term “feeding” means that the feed passes from a conduit or vessel directly to an object without passing through an intermediate vessel.

[0026] The term “passing” includes “feeding” and means that the material passes from a conduit or vessel to an object.

[0027] As used herein, the term “kilopascal” may be abbreviated “kPa” and the term “megapascal” may be abbreviated “MPa”, and all pressures disclosed herein are absolute.

[0028] The following detailed description is merely exemplary in nature and is not intended to limit the various embodiments or the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

[0029] An embodiment of a process and apparatus for purifying an impure gas stream is addressed with reference to a process and apparatus 100 for purifying the impure gas stream predominantly containing hydrogen as shown in FIG. 1. The process and apparatus 100 includes a multilayer adsorption bed 102, a feed stream 104 including the impure gas stream, a feed end 106, a first adsorbent section 108, a second adsorbent section 110, a third adsorbent section 112, a product end 114 and a product stream 116 including purified gas stream. The feed stream 104 is passed to the multilayer adsorbent bed 102 through the feed end 106. The feed stream 104 can be obtained from various sources. In one example, the feed stream 104 is an impure gas stream predominantly containing hydrogen obtained from steam reforming of light hydrocarbons, such as natural gas. In accordance with an exemplary embodiment, the feed stream 104 comprises predominantly hydrogen along with CO₂ and other impurities. Examples of the impurities present in the feed stream 104 include N₂, H₂O, CH₄ and CO. In one example, the feed stream 104 includes about 10 to about 20 mol % CO₂, about 1% to about 10 mol % CH₄, about 1 to about 10 mol % CO and no more than about 2 mol % N₂.

[0030] The feed stream 104 passes through the first adsorbent section 108. The first adsorbent section 108 includes an activated carbon layer and occupies a volume fraction of about 20 to about 50% of the multilayer adsorbent bed 102. In accordance with an exemplary embodiment, the first adsorbent section 108 occupies a volume fraction of about 45% of the multilayer adsorbent bed 102. The first adsorbent section 108 adsorbs at least one of CO₂ and CH₄ present in the feed stream 104. In accordance with various embodiments, one or more layers may be present in the first adsorbent section 108 for removal of H₂O in the feed stream 104. In one example, the first adsorbent section 108 further includes at least one of a silica gel layer and an activated alumina layer upstream of the activated carbon layer in the first adsorbent section 108.

[0031] Subsequently, the feed stream 104 passes to the second adsorbent section 110. The second adsorbent section 110 includes a layer of a molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.5 to 8.0. In accordance with an exemplary embodiment, the second adsorbent section includes a NaY zeolite molecular sieve. The second adsorbent section 110 occupies a volume fraction of about 10 to about 25% of the multilayer adsorbent bed 102. In accordance with an exemplary embodiment, the second adsorbent section 110 occupies a volume fraction of about 15% of the multilayer adsorbent bed 102. The second adsorbent section 110 adsorbs at least CO₂ remaining in the feed stream 104 after passing through the first adsorbent section 108.

[0032] Thereafter, the feed stream 104 passes to the third adsorbent section 112. The third adsorbent section 112 includes a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.0 to 1.5. In accordance with an exemplary embodiment, the third adsorbent section 112 includes one of NaX molecular sieve and a CaX molecular sieve and combination thereof. In accordance with another exemplary embodiment, the third adsorbent section 112 includes one of a Linde Type A Zeolite in addition or in alternative to a NaX zeolite molecular sieve and a CaX zeolite molecular sieve and combinations thereof. In one example, the Linde Type A Zeolite is a 5 A molecular sieve. The third adsorbent section 112 occupies a volume fraction of about 30 to about 60% of the multilayer adsorbent bed 102. In accordance with an exemplary embodiment, the third adsorbent section 112 occupies a volume fraction of about 40% of the multilayer adsorbent bed 102. The third adsorbent section 112 adsorbs at least CO and N₂ present in the feed stream 104. After passing of the feed stream 104 through the various adsorbent sections, the product stream 116 including the purified gas stream is obtained from the product end 114. In accordance with the instant embodiment as discussed, the product stream 116 is purified hydrogen product with purity greater than about 99.9 mol %. In one example, the purified hydrogen product comprises no more than 10 ppmv CO.

[0033] Additionally, once the adsorbent bed becomes saturated with adsorbed impurities, the multilayer adsorbent bed 102 undergoes a regeneration step to desorb at least one N₂, CO₂, CH₄ and CO from the multilayer adsorbent bed 102. The desorption is performed by a reduction in pressure to a pressure lower than the adsorption pressure and by counter-current purging with purified hydrogen at low pressure. The pressure is then increased to the adsorption pressure prior to the next cycle of purification of impure gas stream.

[0034] It should be appreciated and understood by those of ordinary skill in the art that various other components such as valves, pumps, filters, coolers, etc. were not shown in the drawings as it is believed that the specifics of same are well within the knowledge of those of ordinary skill in the art and a description of same is not necessary for practicing or understating the embodiments of the present invention.

[0035] The following is an example of the hydrogen purification process, in accordance with an exemplary embodiment, that is similarly configured to the process and apparatus 100 illustrated in the FIG. 1. The example is provided for illustration purposes only and is not meant to limit the various embodiments of apparatuses and methods for purifying impure gas streams in any way.

[0036] In an exemplary case study, a comparison was made of the multilayer adsorbent bed prepared according to the instant invention, wherein the first adsorbent section included an activated carbon layer occupying a volume fraction of 45%, the second adsorbent section included a NaY zeolite molecular sieve occupying a volume fraction of 15%, and the third adsorbent section having a NaX zeolite molecular sieve occupying a volume fraction of 40% of the multilayer adsorbent bed, with a reference bed. The reference bed included an activated carbon layer occupying a volume fraction of 65% and a NaX zeolite molecular sieve layer occupying a volume fraction of 35% of the reference multilayer bed. A steam reformer feed gas including predominantly hydrogen was passed through the multilayer adsorbent bed in accordance with the instant invention and through the reference bed. Applicants found that using a NaY sieve in the instant multilayer adsorbent bed has a higher equilibrium loading for CO₂ compared to activated carbon as shown in FIG. 2. In this section of the bed, the working capacity i.e. pounds of CO₂ adsorbed per cubic foot of loaded adsorbent as represented on the Y-axis is measured against CO₂ partial pressure between 1 psia and 10 psia represented on X-axis in FIG. 2. As shown in the Figure, the working capacity of the NaY sieve, represented by the curve corresponding to Y-54, is 2.5 times greater than the commercial activated carbon represented by the curve corresponding to H-2-12 in FIG. 2. This provides improved CO₂ polishing and provides better protection for the NaX section against CO₂ migration. As a result, the carbon section was decreased as compared to the reference bed and more NaX sieve can be used in the instant multilayer adsorbent bed. This improves the overall performance of the hydrogen PSA unit, i.e. higher hydrogen recovery at a given product purity specification of 10 ppmv CO. Measured values of the CO₂ removal efficiency during cyclic operation in PSA pilot plant are given in Table 1 below, where the improved performance of NaY compared to activated carbon is evident.

[0037]

TABLE 1
Measured CO2 concentration
(mol %) in pilot plant bed during
PSA cycle, corresponding to the
bed loadings of the instant example
(maximum values during PSA cycle).
Axial Bed	Activated
Position, %	Carbon	NaY
54	4.8	0.12
63	0.29	—

[0038] The measured performance with the bed loading according to multilayer adsorbent bed according to the instant invention compared to the reference bed loading is represented in FIG. 3. The gain in recovery is very significant with the multilayer adsorbent bed of the instant invention as compared to the reference bed as is clear from curves representing NaY and activated carbon, respectively in the figure.

[0039] While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.

[0040] A first embodiment of the invention is a multilayer adsorbent bed for purifying a feed stream comprising CO₂, at least one of N₂, H₂O, CH₄ and CO, and more than 50 mole % hydrogen comprising a) a first adsorbent section comprising an activated carbon layer and occupying a volume fraction of about 20 to about 50% of the multilayer adsorbent bed; b) a second adsorbent section comprising a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.5 to 8.0 and occupying a volume fraction of about 10 to about 25% of the multilayer adsorbent bed; and c) a third adsorbent section comprising a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.0 to 1.5 and occupying a volume fraction of about 30 to about 60% of the multilayer adsorbent bed wherein the second adsorbent section is present between the first adsorbent section and the third adsorbent section. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the first section, the second section and the third section occupy a volume fraction of about 45%, about 15% and about 40% respectively, of the multilayer adsorbent bed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the first adsorbent section further comprises at least one of a silica gel layer and an activated alumina layer. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the molecular sieve in the second adsorbent section is a NaY zeolite molecular sieve. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph, wherein the molecular sieve in the third adsorbent section is one of a NaX zeolite molecular sieve and a CaX zeolite molecular sieve.

[0041] A second embodiment of the invention is a pressure swing adsorption process for purifying a feed stream comprising CO₂, at least one of N₂, H₂O, CH₄ and CO, and more than 50 mole % hydrogen, the method comprising passing the feed stream at an adsorption pressure of about 1500 kPa to about 3500 kPa, through a multilayer adsorbent bed comprising a first adsorbent section, a second adsorbent section downstream from the first adsorbent section and a third adsorbent section downstream from the second adsorbent section, the first adsorbent section comprising an activated carbon layer, the second adsorbent section comprising a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.5 to 8.0 and the third adsorbent section comprising a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.0 to 1.5, wherein the at least one of the N₂, CO₂, CH₄ and CO is adsorbed from the feed stream and a purified hydrogen product is recovered from the multilayer adsorbent bed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the first adsorbent section, the second adsorbent section and the third adsorbent section occupy a volume fraction of about 20 to about 50%, about 10 to about 25%, about 30 to about 60% respectively, of the multilayer adsorbent bed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the first adsorbent section, the second adsorbent section and the third adsorbent section occupy a volume fraction of about 45%, about 15% and about 40% respectively, of the multilayer adsorbent bed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the molecular sieve in the second adsorbent section is a NaY zeolite molecular sieve. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the molecular sieve in the third adsorbent section is one of a NaX zeolite molecular sieve and a CaX zeolite molecular sieve. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the purity of the purified hydrogen product is greater than about 99.9 mol %. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the feed stream comprises about 10 to about 20 mol % CO₂, about 1% to about 10 mol % CH₄, about 1 to about 10 mol % CO and no more than about 2 mol % N₂. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph, wherein the first adsorbent section adsorbs at least one of CO₂ and CH₄, the second adsorbent section adsorbs CO₂ and the third adsorbent section adsorbs at least one of CO and N₂ present in the feed stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising a regeneration step to desorb the at least one N₂, CO₂, CH₄ and CO from the multilayer adsorbent bed, wherein the desorption is performed by a reduction in pressure to a pressure lower than the adsorption pressure and by counter-current purging with a purified hydrogen stream.

[0042] A third embodiment of the invention is a pressure swing adsorption apparatus for purifying a feed stream comprising CO2, at least one of N₂, H₂O, CH₄ and CO, and more than 50 mole % hydrogen, wherein the apparatus comprises a feed end for the introduction of the feed stream thereto, a product end for the recovery of purified feed stream therefrom and a multilayer adsorbent bed comprising a first adsorbent section, a second adsorbent section downstream from the first adsorbent section and a third adsorbent section downstream from the second adsorbent section, the first adsorbent section comprising an activated carbon layer, the second adsorbent section comprising a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.5 to 8.0 and the third adsorbent section comprising a layer of molecular sieve of the faujasite structure type with a Si/Al atomic ratio of from 1.0 to 1.5. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph, wherein the first adsorbent section, the second adsorbent section and the third adsorbent section occupy a volume fraction of about 20 to about 50%, about 10 to about 25%, about 30 to about 60% respectively, of the multilayer adsorbent bed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph, wherein the first adsorbent section, the second adsorbent section and the third adsorbent section occupy a volume fraction of about 45%, about 15% and about 40% respectively, of the multilayer adsorbent bed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph, wherein the first adsorbent section further comprises at least one of a silica gel layer and activated alumina layer. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph, wherein the molecular sieve in the second adsorbent section is a NaY zeolite molecular sieve. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph, wherein the molecular sieve in the third adsorbent section is at one of a NaX zeolite molecular sieve and a CaX zeolite molecular sieve.

[0043] Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

[0044] In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by volume (mole), unless otherwise indicated.